Surveillance of ground topography is well known in the art. In ground surveillance, it is highly desirable to detect whether there has been a material failure in a man-made object such as a road, a pipeline, an electrical grid, or another man-made structure of practical interest. When a structural failure is detected, proper authorities make a determination whether remedial action is necessary. Often times a land-based crew conducts a visual inspection of the ground topography to determine if there is a material failure by traversing an area by vehicle or foot. It is frequently the case that an aircraft or a satellite includes an image capture device such as a charge coupled device (CCD), complementary metal oxide semiconductor device (CMOS) or a radiation detector, such as an infrared sensitive detector. It is well known that airborne photographic systems can also be used for capturing images of adjacent areas of the ground.
When electromagnetic radiation, interacts with matter several phenomena may occur, including scattering, absorption, transmission and reflection of the electromagnetic radiation. Spectral or spectroscopic analysis includes carefully examining, analyzing, and representing the interactions involving electromagnetic radiation and matter, in an orderly fashion, as a function of wavelength, frequency, or time. During spectroscopic analysis, different materials exhibit different scattering, absorption, reflection and transmission characteristics. These distinctive characteristics are determined by the chemical and physical structure of the materials. When a set of these distinctive characteristics are determined to a given level of certainty, as with the use of known test subjects, these spectroscopic results may be referred to as reference spectral signatures or reference spectra.
Natural gas, characteristically, contains a mixture of methane, ethane, and small amounts of other gases. Gas generated by the decomposition of organic matter, henceforth, referred to as swamp gas, only contains methane. It is highly desirable for any natural gas detection method to be able to distinguish between gases released as a result of a failure in a pipeline or a holding container versus emanating swamp gases, thus avoiding false alarms.
Oil pipelines contain significant concentrations of volatile dissolved gas compounds, including methane, ethane, and propane. Oil pipelines operate under pressure; leaks and a concomitant pressure drop result in escaping volatile components, and thereby provide a means for leak detection. Electromagnetic radiation can be directed onto a test subject by any of a variety of means. Commonly, lasers are used but other means such as the use of antennas for radio and microwave electromagnetic energy may be used. Hereafter, when electromagnetic radiation is directed onto a test subject it is referred to as an illuminant.
In detecting failures of gas and oil pipelines there is a particular problem, as the gas or oil pipeline is typically buried beneath ground level. In such cases, it is difficult to make a direct visual assessment of any failures in the pipeline. When failures do occur they are manifest by the leakage of the pipeline contents, the leaking material produces a characteristic trace or signal. Typically, failures in pipelines are currently determined by having personnel walk the pipeline on a periodic and costly basis with some means to detect the trace emanating from the pipeline. Gases can escape a pipeline and travel through subterranean earth to the earth's surface and then into the atmosphere. Consequently, the atmosphere can be monitored for gases that have escaped the pipeline. An association of gases detected in the atmosphere with a pipeline leak may be direct or indirect. An example of a direct association is the release of specific hydrocarbon gases to the atmosphere from subsurface oil and gas pipelines. Natural gas consists of 2 primary components, methane and ethane, with a fairly fixed proportion in a mixture. Measurement of both components and confirmation of the appropriate concentration ratio directly establishes the presence of a pipeline leak. In this case, association is direct in that the gas components themselves are emitted into the atmosphere, albeit with a potentially modified composition. Similarly, other volatile components of the contents of gas-bearing pipelines are detectable and will indicate the presence of a leak. Methane is produced from thermal or biological breakdown of coal. The gas detected (methane) is not the same as the natural resource (coal), so the term “indirect” is used to describe this association. The term “indirect association” does not imply that the scientific basis for the association is weak. The process of converting coal to methane is well described in the scientific literature.
For oil or petroleum pipelines, release of certain volatile components can indicate the presence of a fluid leak, and thus constitute indirect evidence of a pipeline failure. Laser absorption spectroscopy (LAS) is a sensitive means for quantifying molecular concentrations in a variety of situations not amenable to other techniques, particularly remote sensing applications. A main advantage of LAS is that the measurement is done “in situ”; this enables rapid measurements with good spatial resolution in a variety of environments. For an absorption experiment, the ratio of the transmitted beam intensity I(v,x) to the initial beam intensity, I0(v,x=0), is related to an absorber concentration, n, by Beer's Law,I(v,x)/I0(v,x=0)=e−nxσ(v)
The molecular cross-section at frequency, v, is denoted by σ(v) and the path length over which the laser travels by x. For any given signal to noise ratio (SNR) for the measurement of I(v,x)/I0(v,x=0), the measurement sensitivity can be increased by increasing the path length. There are a number of prior art patents that include laser means for detecting trace gases in the atmosphere. Some of these laser-based systems operate in the microwave or the ultraviolet wavelength region. These laser-based systems are unlike the subject invention that operates in the mid-infrared wavelength range. The following patents are discussed since the laser-based systems described therein also operate in the mid-infrared wavelength region while detecting hydrocarbon gases.
In U.S. Pat. No. 4,450,356 issued to Murray et al., a frequency-mixed carbon dioxide (CO2), laser beam is used for remote detection of gases in the atmosphere. The laser beam system uses frequency doubling and frequency summing in crystals to produce wavelengths near three micrometers. Means for selecting many wavelengths are disclosed, but delivery of only two mid-infrared wavelengths to a topographic target are disclosed. CO2 lasers are continuously not tunable and lack strong lines at wavelengths coincidental with acceptable methane and ethane lines. In U.S. Pat. No. 4,489,239, a 25 meter short distance portable remote laser sensor is described for detecting methane gas pipeline leaks by Grant et al. The system requires the use of two separate helium-neon (He—Ne) lasers. The two lasers operate at two different on and off methane signature wavelengths, each of which is fixed. He—Ne lasers are typically not tunable and not as efficient and reliable as solid-state lasers. Similarly, In U.S. Patent Application Publication 2003/0030001 A1, Cooper et al disclose the use of a tunable diode laser to detect gases in the atmosphere. This system does not allow for real-time compensation for variability in the background target reflectivity and cannot measure multiple gas species nearly simultaneously, a critical requirement for scanning and remote sensing systems that detect pipeline leaks. In U.S. Pat. No. 4,871,916, a laser system is described by Scott that uses neodymium lasers for remote sensing of methane in the atmosphere to detect conditions approaching dangerous or explosive levels in a mine. In this system, the wavelength region is nearly at 1.318 micrometers. This system only discloses detection of methane and does not allow for real-time compensation for variability in the background target reflectivity. In U.S. Pat. Nos. 5,157,257 and 5,250,810 assigned to Geiger, a mid-infrared DIAL system is described. This specific system uses six distinct coherent beams formed by six different pulsed lasers at wavelengths 2.2 to 2.4 or 3.1 to 3.5 micrometers to detect light hydrocarbons. The six coherent beams are fully time-multiplexed and combined into a single beam through selective polarization. Quartz crystals are used for polarization. The quartz crystals are easily damaged by high-energy laser pulses and complexity of this system is not conducive to use in the field, particularly in airborne remote sensing applications. Also, the laser spectral width is too broad to resolve the absorption bands of many key gases. In U.S. Pat. No. 6,509,566 B1 assigned to Wamsley et al., a mid-infrared DIAL system is also described for the purposes of oil and gas exploration. The system disclosed includes a single Cr:LiSAF laser with a hydrogen Raman cell to produce wavelengths in a range suitable for hydrocarbon detection. The laser is water-cooled and continuously tunable at a single wavelength. This system does not conveniently allow for real-time compensation for variability in the background target reflectivity and simultaneous detection of other gases. Furthermore, the single laser frequency is referenced to an external frequency meter and is, therefore, subject to drift that negatively affects the electronic components in the system.